In 1983, a mutation that could revolutionize the lamb industry appeared in a commercial sheep flock in Oklahoma. Sheep expressing this mutation exhibited marked enlargement or hypertrophy of certain muscles, notebly those of the hind legs and loin.

Due to the characteristic shape of the muscles of the hind legs, the mutation was named Callipyge (Greek for well-proportioned buttocks). This effect is illustrated in the accompanying photo, with two normal lambs on the left and two Callipyge lambs on the right.

Callipyge lambs quickly began winning ribbons at livestock shows across the country. Feeders were also happy with the improved weight gain and feed efficiency, and packers were impressed with the 30 to 50% higher meat yields.

However, as time went on, a serious draw back became evident: the meat was very tough.

Current research at UC Davis by Department of Animal Science Assistant Professor Bob Sainz, in collaboration with the USDA, Washington State University, University of Idaho, Texas Tech, Utah State University and Superior Packing Co., is aimed at learning about the mechanisms causing the increased toughness and finding ways to overcome it.

The most likely culprit appears to be reduced fragmentation of muscle fibers during the postslaughter aging process due to elevated levels of calpastatin, a protease inhibitor present in muscle. Several potential tenderizing procedures are being investigated, including:

Electrical stimulation, which accelerates the aging process;

Calcium chloride injection which stimulates myofibril fragmentation;

Freezing and thawing, which damages the calpastatin molecule;

The Hydrodyne method, which uses an explosive charge to mechanically disrupt muscle fibers.

Overcoming the tenderness problem would enable use of the Callipyge gene to improve the profitability of the lamb industry